The long-term objective of our research is to understand how cardiac ion channels are modulated by pharmacological agents and by pathological conditions. This information is critical for the design of efficacious antiarrhythmic agents and effective management of cardiac arrhythmias under pathological conditions. The key to achieving this objective is to have a thorough understanding of the structure-function relationship of cardiac ion channels. The advance of molecular cloning and mutagenesis techniques has allowed us to correlate cloned channel subunits with native channels in the heart, and to manipulate their primary sequences in specific ways so that the relation between channel structure and function can be deduced. The focus of this application is the rapid (IKr) and slow (IKs) delayed rectifier K channels in the heart. IKr and IKs are the primary determinants of action potential duration (APD) in cardiac myocytes. Their importance in maintaining the normal cardiac electrical activity is related to their unique gating properties. The fast inactivation and reactivation processes of IKr cause an inward rectification in its I-V. This helps maintain a positive plateau phase during phase 2 and yet sufficient outward current for phase 3 repolarization. On the other hand, the slow activation and deactivation processes of IKs are an important mechanism for APD regulation by changes in the heart rate. These slow gating processes are related to the interactions between two subunits of IKs: KvLQT1 and hIsK. IKr and IKs are also important targets for K channel modulators. Information about the mechanisms and sites of actions of currently available K channel modulators on IKr and IKs may aid the design of new antiarrhythmic drugs. We propose to use cloned K channel subunits as a model (hERG for IKr and KvLQT1/hIsK for IKs), and combine site- directed mutagenesis and electrophysiological techniques to study the structure-function relationship and drug-channel interactions. Four Specific Aims are proposed: (1) To study the structural basis for the unique gating behavior of hERG, (2) To study the mechanisms of azimilide s agonist and antagonist actions on hERG, (3) To study the domains of KvLQT1 involved in interactions with hIsK, (4) To study the mechanisms of differential actions of azimilide and quinidine on IKs. It is anticipated that results from these studies will improve our understanding of the function and modulation of IKr and IKs, and the studies on azimilide and quinidine should provide far-reaching implications for drug actions on K channels.